The Anchored Flexibility Model in LC8 Motif Recognition: Insights from the Chica Complex.

LC8 is a dimeric hub protein involved in a large number of interactions central to cell function. It binds short linear motifs--usually containing a Thr-Gln-Thr (TQT) triplet--in intrinsically disordered regions of its binding partners, some of which have several LC8 recognition motifs in tandem. Hallmarks of the 7-10 amino acid motif are a high variability of LC8 binding affinity and extensive sequence permutation outside the TQT triplet. To elucidate the molecular basis of motif recognition, we use a 69-residue segment of the human Chica spindle adaptor protein that contains four putative TQT recognition motifs in tandem. NMR-derived secondary chemical shifts and relaxation properties show that the Chica LC8 binding domain is essentially disordered with a dynamically restricted segment in one linker between motifs. Calorimetry of LC8 binding to synthetic motif-mimicking peptides shows that the first motif dominates LC8 recruitment. Crystal structures of the complexes of LC8 bound to each of two motif peptides show highly ordered and invariant TQT-LC8 interactions and more flexible and conformationally variable non-TQT-LC8 interactions. These data highlight rigidity in both LC8 residues that bind TQT and in the TQT portion of the motif as an important new characteristic of LC8 recognition. On the basis of these data and others in the literature, we propose that LC8 recognition is based on rigidly fixed interactions between LC8 and TQT residues that act as an anchor, coupled with inherently flexible interactions between LC8 and non-TQT residues. The "anchored flexibility" model explains the requirement for the TQT triplet and the ability of LC8 to accommodate a large variety of motif sequences and affinities.